The present invention relates to a device for metering the blast air delivered to a driven machine, particularly the sheet separation device of a sheet feeder, with a control slide connected via an air intake opening to a pressure source; this control slide has an air discharge opening connected to the driven machine and leading into the open; the opening can be controlled or disconnected during the upward movement or stoppage of the machine coupled with the driven machine by means of a control element actuated as a function of the machine speed.
An arrangement of this type during the starting process of, e.g., a sheet feeder makes possible adaptation to the requirement of the air blast delivered to the sheet separator. An immediate application to the sheet separator of the full amount of air adapted to the standard operating speed would have the result that the top sheets of the sheet stack during the start-up process would not only be loosened, but would be blown all over.
An arrangement of the initially described type is already known. Compared to the previously known manually actuated adjustment device, this known device represents considerable progress. However, various disadvantages of the known devices are caused by the unfavorable air delivery in the region of the control slide, which is formed here by a cylinder provided with radial air connections and a double-piston which can move inside it and has a control groove. The air intake opening formed by a radial bore and the two air discharge openings are diametrally opposite and are offset in the axial direction in such a way that the control groove in the end positions of the double piston connects each time only one air discharge opening with the air intake opening and the other air discharge opening is cut off, except for a small advance air gap. Based on this axial offset, the air current undergoes a considerable deflection corresponding to the offset which may lead to considerable flow losses. The flow around the connection between the two piston portions suggests an additional increase of the flow losses. Such flow losses have a negative effect particularly during the normal operation which requires complete air delivery to the driven machine. This was accomplished in the past by enlarging the air pumps to compensate for the steady losses. However, this leads not only to a simultaneous increase of the power consumption, but also to increased space requirement and weight and thus not only an increase in operating costs, but also an increase in manufacturing costs. The disadvantages involved is seen in the evidently poor efficiency.
There is already known another compressed air control device for sheet feeders where an air line is closed by means of a valve (slide) which is slid across the line cross-section as a function of the machine rpm. The flow in the air line with the slide pulled out does not experience deflection. In contrast to the initially discussed arrangement, the known arrangement in the lower speed range does not experience a division of the air into a partial flow leading to the driven machine and a remaining partial flow leading to the outside. Therefore, it is possible that during the starting process in the area of the throttle valve (slide) projecting into the line cross-section there results a considerable air excess and hence a corresponding air stagnation which affects the pump ahead of it and may lead to overloading the pump and the entire pump driven which again requires "beefing up" (overdimensioning) and may have a negative effect on the life and the efficiency. Aside from this, DL-PS- 85 367 involves only schematic sketches. The practical execution of the afore-mentioned slide arrangement leads to arrangements which, as the initial arrangement, involve deflection and flow losses.
It is, therefore, an object of the present invention to provide an arrangement of the initially described type where during normal operation line losses in the area of the control slides are avoided and which can be equipped with relatively light pumps whose manufacturing and operating costs are low and whose overloading is effectively prevented.
Another object of the present invention is to provide an arrangement of the foregoing character which may be easily maintained in service and has a substantially long operating life.